1. Field of the Invention
The invention relates to a positioning system for positioning a member and to a record medium for use in such a system. The invention also relates to data storage apparatus employing a transducer positioning system.
2. Description of the Prior Art
In U.S. Pat. Nos. 3,534,344 (Santana) and 3,691,543 (Mueller), there are disclosed positioning systems for positioning data transducers to selected tracks on a stack of magnetic recording disks. The data transducers are ganged for movement with a servo transducer which derives position information from servo tracks pre-recorded on one of the recording disks.
The servo tracks in U.S. Pat. No. 3,691,543 are concentric and each consists of relatively long portions of track magnetised in one direction alternating with relatively short portions magnetised in the opposite direction. Magnetic transitions in one of the two directions are aligned from one track to the next and since the tracks are written across the disk surface with no gaps between them, continuous transitions of the same polarity extend radially across the disk surfaces at regularly spaced intervals.
The magnetic transitions in the opposite direction are staggered from one track to the next. In other words, if a section of a track between two continuous transitions (referred to as a servo cell), consists of a relatively long portion followed by a relatively short portion, then the corresponding servo cell on an adjacent track consists of a relatively short portion followed by a relatively long portion and vice-versa. With this arrangement, data heads are said to be on-track when the servo head is centrally located over the boundary, or guidepath as it is called, between two adjacent servo tracks.
If the data head moves off-track, an error signal is developed by the servo head as it moves from its guide path mid-way between adjacent servo tracks to lie more over one track than the other. The magnitude and polarity of the resulting error signal indicates the degree and direction of the off-set of the data head from the on-track position. The servo head and actuator are connected in a closed loop servo system which responds during a track following operation to energize the actuator so as to maintain the data head accurately on-track with zero error signal.
During track accessing operations, the servo error signal alternates in polarity as tracks are crossed and is used to provide track crossing pulses needed to determine the actual position of the data head. An indication of the instantaneous speed of the data head during an access operation is derived from the rate of change of error signal. This is used to control the actuator so that the data head follows a desired velocity profile during an access operation.
Since the error signal in the arrangement described in U.S. Pat. No. 3,691,543 is linear only for about a quarter of a track either side of the on-track position, in practice it is combined with a further signal, also proportional to velocity, obtained from the current supplied to drive the actuator. The combined signals provide a fairly accurate indication of the actual speed of the head during the track access.
In U.S. Pat. No. 3,902,116 (Palmer) there is described an electronic tachometer for generating a continuous velocity output signal from the combination of two quadrature incremental position signals. Such signals are derived from an optical system in which a movable grating on a member to be positioned modulates light transmitted through a fixed grating. The light is detected by two light detectors which are placed so that they produce output signals which are 90.degree. out of phase. The Palmer patent does not show the application of such a scheme to a positioning system in which position reference information is recorded in tracks extending in a direction of intended motion. Nor does the patent show how a single position transducer can provide quadrature position signals in cooperation with an appropriate position reference pattern recorded on a medium.
In some very early transducer positioning systems for disk files such as are described in U.S. Pat. No. 3,034,111 to Hoagland and in an article entitled "Positioning System" by J. O. Hildebrand in the IBM Technical Disclosure Bulletin Vol. 3, No. 11, page 57 (April 1961), there are shown so-called "checkerboard" servo patterns, sections of which may be divided into even and odd sectors, laterally offset from each other by half the width of a square. A servo transducer could be positioned to any row boundary by nulling its output signal. When on a boundary, the servo transducer simultaneously detected equal and opposite magnetic transitions. Great precision was required to record opposite transitions in adjacent tracks in absolute longitudinal alignment so that false position error signals were not generated. Furthermore, to sample and detect the checkerboard transitions, a separate timing track and transducer were required. Offset sections were used to provide positioning at a greater track density, but were not used to produce simultaneous out of phase incremental position signals.